Enzyme conversions are typically carried out in a batch or continuous aqueous slurry process. Typically, a "high solids" enzyme slurry contains about 18-35 wt. % solids. The conversion is carried out as the granular starch is being heated (to gelatinize it) so that the high viscosity of the native starch is not reached and agitation can be maintained in the high solids mixture. After the enzyme conversion is completed, the enzyme is inactivated. The converted starch is often diluted prior to use.
Some of the patents covering enzyme conversion or acid and enzyme conversion are discussed below.
U.S. Pat. No. 2,609,326 (issued Sep. 2, 1952 to W. W. Pigman et al.) discloses rapidly gelatinizing and dispersing starch granules in hot water while subjecting the starch to intense agitation and shearing, immediately converting the gelatinized and dispersed starch at an elevated temperature with a starch-liquefying amylase characterized by its ability to hydrolyze the starch molecules into large fragments, inactivating the enzyme, and immediately drying the enzyme converted starch.
U.S. Pat. No. 3,560,343 (issued Feb. 2, 1971 to F. C. Armbruster et al.) discloses a process where a starch is acid hydrolyzed to a D.E. less than 15 and then converted with a bacterial alpha amylase to a DE between 10 and 25.
Japanese 46-14706 (published Apr. 20, 1971) discloses a continuous process for preparing a granular converted starch which swells, but does not dissolve in cold water, and which is reduced in viscosity. A starch alpha amylase mixture having a water content of 40-60%, containing buffer to adjust the pH to 5-7, is cured for several hours at room temperature, or a temperature at or below the gelatinization temperature, after which it is put into a starch dryer maintained at 70-150.degree. C. During the drying, the temperature and water content change to those suitable for hydrolyzing the starch. The hydrolysis, drying of the hydrolyzed starch, and deactivation of the residual enzyme simultaneously occur during the heating at 70-150.degree. C. A liquefaction-type amylase shows the strongest hydrolytic activity at 70-90.degree. C., but at higher temperatures (i.e., above 90.degree. C.), if the moisture content is above 35%, the starch undergoes the hydrolytic activity but is gelatinized at the same time and if the water content of the mixture is less than 30%, it becomes more difficult to gelatinize the starch, but at the same time the hydrolysis by the enzyme shows a tendency to fall off rapidly. To satisfy these opposing tendencies, it is necessary to reduce the water content of the mixture from 40-60% to 30-35% in the dryer and to increase the temperature to 90-100.degree. C. during the enzyme hydrolysis.
U.S. Pat. No. 3,663,369 (issued May 16, 1972 to A. L. Morehouse et al.) discloses a two-stage hydrolysis. The first stage is carried out with acids or enzymes at elevated temperatures for short periods to liquify the starch with very little dextrinization or saccharification. The second stage is carried out at an alkaline pH with bacterial alpha amylase to achieve the desired D.E.
U.S. Pat. No. 3,644,126 (issued Feb. 22, 1972 to D. A. Bodnar) discloses a two step of making a starch conversion syrup by treating an aqueous slurry of starch with a starch-liquefying enzyme under conditions sufficient to give a product with a D.E. of &lt;35. The liquefied starch slurry is then digested with glucoamylase and malt enzymes to obtain a syrup having &lt;45% dextrose and sufficient maltose to provide at 85% total fermentable sugars.
U.S. Pat. No. 3,849,194 (issued Nov. 19, 1974 to F. C. Armbruster) discloses treating a waxy starch with a bacterial alpha amylase at a temperature above 85.degree. C. to liquify the waxy starch, cooling the liquified waxy starch to about 80.degree. C., and converting the liquified waxy starch with the bacterial alpha amylase to a D.E. of from about 5 to about 25.
U.S. Pat. No. 3,853,706 (issued Dec. 10, 1974 to F. C. Armbruster) discloses hydrolyzing starch with a bacterial alpha amylase to a DE of less than 15, terminating the hydrolysis by heat treatment, and further converting to a DE of between about 5 and 20.
U.S. Pat. No. 3,922,196 (issued Nov. 25, 1975 to H. W. Leach) discloses hydrolyzing an aqueous slurry of starch (5 to about 40% solids) with alpha amylase and optionally a saccharifying enzyme (e.g., beta amylase or glucoamylase). The saccharifying enzyme is preferably added after the granular starch is substantially solubilized at the solubilization temperature or a lower temperature (e.g., 50-65.degree. C. and pH 4-6). The mixture is heated at a temperature between the initial gelatinization temperature and the actual gelatinization temperature of the starch, preferably at a pH of 5-7.
U.S. Pat. No. 4,014,743 (issued Mar. 29, 1977 to W. C. Black) discloses a method for the continuous enzyme liquefication of starch. Preferably, the starch is a raw starch. A suitable enzyme is bacterial alpha amylase. An enzyme-containing suspension of the starch (10-45 wt. % on a dry solids basis) is continuously added to an agitated body of the heated converted starch (170-210.degree. F.). The incoming starch is gelatinized and mixed with the partially converted starch to maintain a blend having a viscosity low enough to be readily agitated and pumped. A stream of the blend is continuously removed from the conversion tank and treated to inactivate the enzyme. The process is controlled to limit the maximum viscosity of the blend to a Brookfield viscosity of not over 5000 cps (100 rpm and 88.degree. C.-190.degree. F.). A blend of starches that have been subjected to different degrees of enzyme conversion is obtained since the heating and enzyme treatment is not uniform for the individual starch granules or molecules.
U.K. 1,406,508 (published Sep. 17, 1975) discloses a continuous process for liquefying natural or chemically modified starch to give starch pastes having a solids content of up to 70% by weight. The starch in granular form, without the intermediate formation of a slurry, is continuously supplied to a reaction zone where it is subjected to the action of an enzyme (e.g., alpha amylase) in a stirred aqueous medium at an elevated temperature (50-98.degree. C.) and pH of 4.5-8. Once the liquefaction is completed the liquefied starch is stabilized by deactivating the enzyme. A greater proportion of large molecules and a broader molecular weight distribution result as compared to a discontinuous process where the molecules are smaller and substantially the same size.
DE 37 31 293 A1 (laid open Apr. 8, 1980) discloses a process for continuously degrading and digesting starch. A dry starch powder together with liquid water or an aqueous starch suspension is charged to a stirred converter containing a starch degrading enzyme, preferably alpha amylase, while the temperature is increased to 70-90.degree. C. by injecting steam at 120-125.degree. C. and 2-4 bar. The product leaving the converter is treated with an enzyme deactivating agent before final dilution to the desired concentration. Starch pastes with a solids content of up to 80% and higher are obtained.
U.S. Pat. No. 4,921,795 (issued May 1, 1990) to F. A. Bozich, Jr.) discloses an improved slurry method for producing dextrin adhesives using alpha amylase in combination with glucoamylase. The function of the glucoamylase is to eliminate the limit dextrin problem and a mechanical shearing step. The alpha amylase randomly cleaves the .alpha.(1.fwdarw.4) linkages of the linear amylose molecules and cleaves the branched amylopectin molecules up to the (1.fwdarw.6) glucosidic linkages of the limit dextrin. The slurry is stirred sufficiently to create a vortex in the aqueous reaction slurry, thereby maintaining adequate mixing without shearing. The hydrolysis is allowed to continue until an optimal mix of fragment sizes is achieved (as indicated by a Brookfield viscosity of 1000-2000 cps at 20 rpm, 110.degree. F., 45-55% solids, and 0 to 16% sodium borate pentahydrate). The enzyme is then inactivated. The Theological properties of the resultant slurry can be adjusted as needed.
EP 231,729 (published Aug. 4, 1993) discloses a two step process for the enzymatic degradation of flour. The first step involves treatment with alpha amylase. The second step involves treatment with beta amylase optionally in combination with pullulanase. The dry solids content of the suspension is as high as possible, e.g., 30-50%.
U.S. Pat. No. 5,445,950 (issued Aug. 29, 1995 to S. Kobayashi et al.) discloses a method for slightly decomposing a granular starch with an alpha amylase and/or glucoamylase at 10-65.degree. C. to reduce the viscosity of the granular starch. The starch is decomposed 0.1-15%, preferably 0.1-1%.
There is a need for a process which can be used to prepare high solids, enzyme-converted starches.